Every day, as many as 2.8 million runaway and homeless youth live on the streets of America. Of the many significant problems they face, substance abuse may be the most detrimental to their long-term health and survival. Relative to housed peers, a staggering two to three times more homeless youth use substances. Over 75% of homeless youth/young adults report lifelong alcohol and/or marijuana use. As many as one-third indicate illicit drug use. These substance use trends are a major health issue, given that the short- and long- term consequences include mental health disorders, cancer, and HIV. Substance use among homeless youth has been linked to child abuse and parental substance abuse, but little is known about how protective mechanisms operate to offset conditions of risk for substance use/abuse for this extremely vulnerable population. The long-term goal of the PI's research program is to improve the lives of homeless youth/young adults by informing the prevention of substance abuse and protection against future health risks. In support of this goal, the specific aims of this study are to: 1) identify the direct association between primary stressor and substance use; 2) elucidate the extent to which protective factors (including psychological and social resources) and secondary stressors (including psychological and social stress since leaving home) mediate the relations between primary stressors and substance use; 3) determine whether protective psychological and social resources moderate the relations between primary stressors and substance use; 4) determine whether secondary stressors and protective social resources co-vary with substance use and change in use over 30 days; and 5) determine the utility and feasibility of short message service (SMS) surveying to gather multiple daily data on substance use, secondary stressors and protective social resources in real time. While some of the risk factors for substance use are understood, there is a dearth of empirical studies examining both primary and secondary stressors and how protective mechanisms mediate or buffer between childhood disadvantage (a primary stressor) and substance use. Additionally, studies that examine resources and stressors that co- vary with substance use are rare. While identifying risk factors for substance use is critical, equally important is understanding why some youth never initiate or are able to abstain from substance use, and identifying unique modifiable social and psychological resources that serve as protective factors against substance use. Data collection methods will include a face-to-face structured interview with 120 homeless youth/young adults through two service agencies, followed by multiple daily text questions sent to respondents over a 30-day period, and conclude with a follow-up structured phone interview. Uncovering protective factors successfully used by homeless young people to abstain from substance use and understanding patterns of change in substance use over time will inform future intervention strategies to improve health outcomes.